The Standard Model of particle physics describes the production, interaction and decay of Nature’s smallest constituents: the elementary particles. Over time and through many experiments, the Standard Model has become one of the most extensively tested theories in physics. Its predictions have been experimentally validated and proven to be impressively precise. However, despite this success, the Standard Model cannot explain some of the phenomena seen in Nature.

One of the most striking problems of the Standard Model is the electroweak hierarchy problem – that is, the fact that the mass of the Higgs boson is much smaller than expected. This could be resolved by the existence of new fundamental interactions and new heavy particles, whose contributions to the mathematical description of the Higgs boson would explain its mass (cancelling divergent radiative corrections). Searches for new particles have therefore always been a core component of the ATLAS physics programme, where a variety of different analysis efforts are taking individual approaches to the search.

But sometimes it is better to combine forces. The ATLAS Collaboration carries out statistical combinations of individual searches in order to exploit the full potential of the available data. Such combinations have a long tradition in the high-energy physics community, as the complementarity of the individual analyses allows researchers to set tighter constraints on new-physics models.

The ATLAS Collaboration has released a new combined search for heavy vector bosons, which includes 13 individual searches studying different final states. The new combination includes, for the first time, a dedicated search for heavy particles decaying to third-generation leptons (taus). Further, unlike previous combinations, all of the searches employ the full LHC Run-2 dataset.

Physicists optimised each analysis independently in order to reach maximal sensitivity. They introduced additional requirements to guarantee that collision events were not used in more than one analysis channel. In addition, researchers gave careful consideration to the design of the combination’s statistical model, extensively studying the correlations between individual channels.

The combination allowed ATLAS researchers to set new limits on the Heavy Vector Triplet (HVT) model – an extension of the electroweak sector that describes new heavy particles interacting with the electroweak force. The interaction strengths of these new particles to Standard-Model quarks, leptons and bosons are free parameters of the HVT model. While individual analyses have limited sensitivity to these parameters, when combined they can set strong constraints on all relevant model parameters. Figure 1 shows the representative exclusion limits on the interaction-strength (coupling) parameters between new heavy vector bosons and Standard-Model fermions, the Higgs boson and W/Z bosons. Figure 2 presents constraints on couplings to leptons and quarks.

By combining numerous analyses, ATLAS researchers have thus developed a powerful search for new heavy particles. Going forward, they plan to expand their combination by including searches for heavy particles decaying into (third-generation) quark pairs. This will further enhance the sensitivity of such combined search efforts, allowing physicists to probe as-yet unconstrained parameters.

Learn more

• Combination of searches for heavy resonances using 139 fb⁻¹ of proton-proton collision data at 13 TeV with the ATLAS detector (ATLAS-CONF-2022-028)
• Combination of searches for heavy resonances decaying into bosonic and leptonic final states using 36 fb⁻¹ of proton-proton collision data at 13 TeV with the ATLAS detector (Phys. Rev. D 98 (2018) 052008, arXiv: 1808.02380, see figures)
• LHCP 2022 presentation by Enrique Kajomovitz Must: Direct searches at LHC in the context of B anomalies (exp: ATLAS, CMS, LHCb)
• Stronger together: combining searches for new heavy resonances, ATLAS Physics Briefing, August 2018
• See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.